Orthogonal frequency division multiplexing (OFDM) is a popular digital modulation scheme in radio frequency (RF) wireless communications. In principle, OFDM uses multiple closely spaced orthogonal subcarriers, and each subcarrier is modulated at a relatively low symbol rate. In practice, OFDM be implemented using fast Fourier transform (FFT) algorithms, and signal processing technologies are further leveraged to improve the OFDM system performance. OFDM has been shown to have strong ability to cope with severe channel conditions (e.g. RF channels with large multi-path fading effect) and have high spectral efficiency. OFDM has been applied in WiMAX, wireless LAN, ADSL, digital radio and video broadcasting systems, and was chosen as a candidate technology in wireless 4G standards.
Recently OFDM over optical fiber transmission has attracted a lot of attention. Optical OFDM systems have been shown to have superior tolerance to fiber CD and PMD. Compared with incoherent optical OFDM, coherent optical OFDM can have higher receiver sensitivity and better transmission performance. However, coherent optical OFDM receivers inherit the many issues facing a traditional coherent optical communication system, including the need of an extra local optical oscillator (laser), the phase noise and frequency drift of the local laser, and the random polarization rotations of the incoming signals. Although some signal processing algorithms have been proposed and demonstrated to effectively mitigate the phase noise and polarization rotation issues in the electrical domain, their demand on high-speed signal processing and computation power becomes increasingly high and makes it highly challenging to implement a real time system at high speed.
Also, system performance of optical OFDM signal transmission over fiber relies heavily on complex digital signal processing (DSP). When the system bit rate is beyond 40 Gb/s, the current DSP processing speed and computation power fall far behind what an optical OFDM system would require. Therefore, it is important to design new architectures of optical OFDM systems, simplify the system and reduce the requirements on DSP circuits.
The diagram 10 in FIG. 1 illustrates the conventional approach for coherent OFDM signal detection of a received signal 11 using a local oscillation (LO) laser 12. The light from LO laser 12 interferes with the incoming optical OFDM signal 11 at the coherent receives 13 and the signal is then demodulated 14. Since the two beating light beams come from two independent light sources, the phase noises of the LO laser and random phase fluctuations of the incoming signal can cause signal degradations.
Accordingly, there is need for a new coherent optical OFDM receiver using self optical carrier extraction.